docs: def pkcs7_pad(buf):
      # type: (bytes) -> bytes
      """Appends PKCS7 padding to an input buffer
      :param bytes buf: buffer to add padding
      :rtype: bytes
      :return: buffer with PKCS7_PADDING
      """
      padder = cryptography.hazmat.primitives.padding.PKCS7(
          cryptography.hazmat.primitives.ciphers.
          algorithms.AES.block_size).padder()
      return padder.update(buf) + padder.finalize()